Search

We present a study of novel clay-polymer nanocomposite materials using a combination of both atomistic simulation and experiment. We show how computational simulation can bring new insight to our understanding of the structure and dynamics of these materials. Experiments of the intercalation of low molecular weight amine functionalized (poly propylene) oxide oligomers into Na + -montmorillonite clay show that we can control the interlayer separation of the organo-clay system to obtain non-exfoliated nanocomposites. Infrared spectroscopy experiments indicate that an increase in hydrogen-bonding occurs when amine and protonated amine monomers are intercalated into the clay galleries. Molecular dynamics simulations offers new information that corroborates these finds by giving a detailed description of the structure of the nanocomposites.

Ab initio calculations have been carried out at the MP2/6-31G ∗ level of theory on the aza-Diels-Alder reaction of a range electron deficient imines (with sulfonyl and/or carbonyl substituents) with buta-1,3-diene. In all cases but one, an early cyclic transition-state could be located in which most often, the forming NC bond was more fully formed than the forming CC bond. However, examples were found in which this order of bond formation was reversed. Examination of HOMO-LUMO energy levels show that aza-Diels-Alder reactions are HOMO (diene)-LUMO (dienophile) controlled and that the most electron deficient imines should be more reactive than simple alkyl or aryl imines and imines possessing less electron withdrawing groups. Evidence is also found for exo -lone pair effects, but these effects become obscured by other stereoelctronic effects as electron withdrawing substituents are added to the imine.

The absorption spectra of two reactive dyes were studied by UV/Visible spectrophotometry in the presence of sodium chloride and urea at varying concentrations. Evidence has been produced to show that the effect of adding common salt to a dyebath at neutral pH is to produce increased aggregation of the dye species. In contrast, the addition of urea to a dye liquor results in reduced dye aggregation and the generation of increased amounts of the monomeric species. ©

Cementitious materials are among those most widely used by mankind while being among the least well understood. The detailed physicochemical processes involved in the hydration and setting of cement slurries are very complex, and a clearly defined quantitative account is still lacking; indeed, even the composition of the cement powder itself is not known exactly. Still less has there been any understanding of the mechanism by which numerous known retarders of the cement setting process act. In this article, we detail the synthesis of novel macrocyclic organophosphonate retarders 1a and 2a which were developed by rational methods. Attempts to synthesise these compounds as phosphonate ester derivatives were universally unsuccessful, however direct modification of the parent hexaaza- (3) and trioxatriaza-18-crown-6 (5) derivatives was successful, to provide the phosphonic acids 1b and 2b respectively. Subsequent testing of these compounds showed their ability to inhibit the growth of crystalline ettringite and delay the setting of cement. These results support the hypothesis that the formation of crystalline ettringite is the rate determining step in the setting of cement.

N,N-dibenzyl-2-aminopropanal 1a derived from alanine rearranges to the corresponding ketone, N,N-dibenzyl-1-aminopropanone upon exposure to either silica gel or pyridinium acetate. Similar rearrangements do not occur with aldehydes derived from phenylalinine, valine, leucine, proline or phenylglycine. One mechanistic explanation for the rearrangement is a facile 1,2-methyl shift, followed by 1,2-hydrogen shift. © 1997 Elsevier Science Ltd.

The dissociation of singly or multiply protonated peptide ions by using low-energy collisional activation (CA) is highly dependent on the sites of protonation. The presence of strongly basic amino acid residues in the peptide primary structure dictates the sites of protonation, which generates a precursor ion population that is largely homogeneous with respect to charge sites. Attempts to dissociate this type of precursor ion population by low-energy CA result in poor fragmentation via few pathways. The work described here represents a systematic investigation of the effects of charge heterogeneity in the precursor ion population of a series of model peptides in low-energy CA experiments. Incorporation of acidic residues in the peptide RLC*IFSC*FR (where C* indicates a cysteic acid residue), for example, balances the charge on the basic arginine residues, which enables the ionizing protons to reside on a number of less basic sites along the peptide backbone. This results in a precursor ion population that is heterogeneous with respect to charge site. Low-energy CA of these ions results in diverse and efficient fragmentation. Molecular modeling has been utilized to demonstrate that energetically preferred conformations incorporate an intraionic interaction between arginine and cysteic acid residues.

Furan and 2-hydroxymethylfuran were reacted under Lewis acidic oligomeric precursors, for a subsequent Lewis acid catalysed cyclisation to afford the furan based calixarenes, i.e.cyclic tetramer2a, and small quantities of the cyclic pentamer2b, hexamer 2c and octamer2d.

This article examines the way in which the English language international news media has constructed the threat of cyberterrorism. Analysing 535 news items published by 31 different media outlets across 7 countries between 2008 and 2013, we show that this coverage is uneven in terms of its geographical and temporal distribution and that its tone is predominantly apprehensive. The article argues that, regardless of the ‘reality’ of the cyberterrorism threat, this coverage is important because it helps to constitute cyberterrorism as a security risk. Paying attention to this constitutive role of the news media, we suggest, opens up a fresh set of research questions in this context and a different theoretical approach to the study of cyberterrorism.

New catalytic asymmetric Lewis acids have been developed for the aza-Diels-Alder reaction of a methyl glyoxalate and p-anisidine derived imine with Danishefsky's diene. The catalysts were developed using parallel combinatorial methods, with the highest e.e. being 97 % using (1S, 2S)-1,2-diphenylethylenediamine and magnesium iodide in acetonitrile.

N-[(1R)-(–)-camphor-10-ylsulfonyl]imine 1b generated in situ from the corresponding brominated glycinate 4b reacts with Danishefsky's diene under thermal conditions to give a high yield of diastereoisomeric Diels–Alder adducts 7b and 6b in a 2.04 : 1 ratio. Under Lewis acid catalysis at –78 °C the diastereoselectivity improves slightly to 2.30 : 1 using titanium tetraisopropoxide. However, the sense of asymmetric induction is reversed when using diethylaluminium chloride as the catalyst. Determination of the absolute stereochemistry of the adducts 6b and 7b was achieved by comparison with pipecolic acid derivatives 10b and 12b and showed that 7b had the (2R)-configuration at the newly formed chiral centre and 6b the (2S)-configuration.

Furan and 2-hydroxymethylfuran reacted under Lewis acidic conditions to yield oligomeric furylmethane compounds 5 and 6. Difurylmethane 5 was also obtained via the reaction of 2-furyllithium with bromochloromethane or reaction with 2-furaldehyde, followed by in situ reduction of the resulting lithium alkoxide with sodium boranuide–trifluoroacetic acid mixture. The trimeric furan compound 6a could also be prepared from furyllithium by a similar route. Oligomers 5 and 6 were utilised for a subsequent Lewis acid-catalysed cyclisation to afford calixfurans, i.e. cyclic tetramer 2a, and small quantities of the cyclic pentamer 2b, hexamer 2c and octamer 2e. Traces of cyclic heptamer 2d could also be detected under certain conditions. Chemical ionisation mass spectrometry using ammonia gas demonstrates that the calixfurans and their acyclic precursors are capable of chelating hydrogen and ammonium ions in the gas phase.

Force field parameters for use in MM2 modelling programs have been developed for the diazoketone function, from a combination of X-ray crystallographic data, IR spectroscopy, and ab initio and semi-empirical calculations. These parameters have been applied to study the accessible conformations and structure of two diazoketone containing molecules, for which X-ray crystallographic data is available. Comparison of X-ray data and calculated geometry was only possible after allowance was made for the effect of crystal packing forces.

E- and Z-1-alkenyl iodides react with vinylboronate pinacol ester 1 under palladium(0) catalysed, Heck conditions to provide polyenylboronates in moderate yields after purification. The addition of either silver(I) or thallium(I) salts to the reaction mixture is essential. Alkenyl bromides are unreactive under identical reaction conditions.

In order to determine whether the remote asymmetric induction in the reduction of compounds such as 1 and 2 using borane is really controlled by intramolecular chelates of type 3, rather than dioxaborolane oxygen-borane chelates of type 12, a study was undertaken to examine related reductions involving the corresponding homochiral acetal 30 and comparative reductions of the dioxaborolane 14 and the acetal 23b. While this study showed that reductions of the dioxaborolane 14 and the acetal 23b with borane and L-Selectride were virtually identical, this result did not necessarily indicate that dioxaborolane oxygens or acetal oxygens were directing borane reduction. However, that the more likely explanation for the remote asymmetric induction observed for 1 and 2 being mediated by complex 3 was confirmed by the fact that the acetal 30 gave no asymmetric induction with borane. A crystal structure of the phenylboronate ester 10 has been carried out.

The first total synthesis of racemic Phthoxazolin A 4 is described, involving a convergent series of palladium cross-coupling reactions to stereoselectively construct the Z,Z,E-trienyl unit. The most important steps involve using vinylboronate pinacol ester 1 as a vinyl dianion equivalent, by employing a Heck coupling of a vinyl iodide 9 with the vinyl boronate 1, followed by a deboronation–iodination sequence with inversion of alkene stereochemistry to provide a new alkenyl iodide 6. Final Stille coupling of the vinyl iodide 6 with an oxazolyl alkenyl stannane 40 provided Phthoxazolin A 4.

MM2 force field parameters have been developed for alkyl- and aryl-boronates and for their carbonyl Lewis-acid complexes, from a combination of X-ray crystallographic data, infrared spectroscopy, ab initio and semi-empirical calculations. The resulting parameters, when used in conjunction with MacroModel, reproduce the geometry of ab initio and MNDO calculated and X-ray crystal structures with high accuracy, and have been used to model chiral boronate esters to explain the origin of stereoselective reduction reactions of boronate complexes 3 and 4.

The ytterbium(III) triflate-catalysed reaction of a para-methoxyaniline, ethyl glyoxalate-derived imine, with a series of different dienes has resulted in products which initially suggest the operation of two possible modes of aza-Diels–Alder reaction, however, a more likely explanation is that a common reaction mechanism is operating, involving a stepwise Lewis acid-catalysed process and a common type of zwitterionic ytterbium complex.

A synthesis of 2, the chromophore of nibrolone (1), is described. The key constructive step is the regiospecific cycloaddition 10 + 11a → 12a.

t-Butyl 2-ethoxycarbonylcarbapen-1-em-3-exo-carboxylate (1a) reacts with osmium (VIII) oxide to give t-butyl (1SR,5SR,7RS,8RS)-1-ethoxycarbonyl-8-hydroxy-2-oxa-6-azabicyclo[3.2.1]octane-7-carboxylate (7a).

The title reaction [(1a)→(10a)] is induced by the action of osmium(VIII) oxide. Presumably, the diol (12), formed by the addition of the elements of hydrogen peroxide to the endo-face of the 1,2-double bond of the precursor (1a), is an intermediate in the reorganisation. The rearrangement product is shown to possess structure (10a)[rather than (11a) as proposed in the preliminary communication of this work] by X-ray crystallography. Compound (10a) undergoes dehydrogenation in the presence of lead(IV) acetate or chromium(VI) oxide to give t-butyl (1RS,5SR,8SR)-8-ethoxycarbonyl-8-hydroxy-3-oxo-2-oxa-6-azabicyclo[3.3.0]oct-6-ene-7-carboxylate (13a). Unusual results are encountered when compound (10a) is subjected to methylsulphonylation conditions. Thus, with methanesulphonyl chloride and triethylamine in dichloromethane, it reacts to give the O-methanesulphonate (10d) as the major product and (depending upon the experimental conditions) either the N-methylsulphonylmethylsulphonyl-O-methylsulphonyl derivative (10f) or the N-methylsulphonylmethylsulphonyl derivative (10h). The last-cited compound is the major product when the oxazabicyclo-octane (10a) is added to an ‘aged’ mixture of methanesulphonyl chloride and triethylamine in acetonitrile. In the presence of methanesulphonyl chloride in pyridine, compound (10a) is transformed into the N-methylsulphonyl derivative (10c). A study of the 300 MHz 1H n.m.r. spectra of oxazabicyclo-octanes of type (10) and oxazabicyclo-octenes of type (13) reveals some unexpected downfield shifts of the 1-hydrogen atoms as a consequence of O-substitution.